The electrical and dielectric properties of paper are important parameters in electrophotography, influencing both toner transfer and runnability in laser printers and in digital printing machines. These parameters are dependent on the ionic content and moisture content of the substrate. The effects of NaCl content of the substrate and the ambient relative humidity (RH) on the direct current resistivity in different electric fields and on the real part of the dielectric constant and dielectric loss factor at different frequencies have been investigated with experimental papers of different grammages. A physical model of electrical conductivity in papers with ionic addition is proposed, based on the hypothesis that ionic addition not only increases the number of available free ions but also changes the water state and the paper structure, which influence the ionic motion in an electric field. This model is supported by the dependencies of the real part of the dielectric constant and the dielectric loss factor on the NaCl content, RH, and frequency.